1. Field of the Invention
This invention relates to fluid separation modules. In another aspect, the invention relates to fluid separation modules having tube sheets positioned in the end portions of the cylindrical shell forming the exterior of the separation module wherein the end portions of the cylindrical shell have been modified in order to provide improved adhesion and fluid seal between the tube sheets and the shell walls.
2. Prior Art
It is known to separate one fluid from a mixture of fluids by using a hollow fiber membrane or bundle of hollow fiber membranes which are more permeable to the one fluid than the other fluids of the mixture. The fluid mixtures are brought into contact with one side of the hollow fiber membranes, with a pressure differential being maintained across the membrane, and the permeable fluid will permeate the membrane to the other side thereof and thereby become separated from the fluid mixture.
Separation modules containing hollow fiber membranes are advantageous in view of the high ratio of membranes surface area per unit volume of the separation module which can be achieved. Consequently, separation modules containing hollow fiber membranes may be of sufficiently compact size to find desirable application for many fluid separation operations such as gas - gas, gas - liquid, and liquid - liquid (including liquid - dissolved solid) separations.
In these fluid separation modules, at least one end of each of the hollow fiber membranes is embedded (often commonly referred to as potted) in a tube sheet, and the hollow fiber membranes extend in a fluid communication relationship through the tube sheet. One purpose of the tube sheet is to ensure the hollow fiber membranes are in an essentially fluid tight relationship within the separation module. The tube sheet may be secured in an essentially fluid tight relationship in the separation module such that fluid does not pass from one of the exterior side or bore side to the other side of the hollow fiber membrane except through the wall of the membrane. Even small leakages around the tube sheet can significantly adversely affect the performance of the separation modules since non-permeating fluids can pass via these leakages to the permeate exit side of the hollow fiber membranes and reduce the selectivity of separation which can be achieved by the separation module. Another purpose of the tube sheet is to provide a significantly strong barrier to fluid flow that during operational conditions, the tube sheet does not rupture or otherwise lose its integrity such that fluid can pass through the tube sheet. In addition, the tube sheet cannot slip or be pushed by pressure differentials of operation. Therefore, the tube sheet is more often of substantial thickness in order to ensure achieving a fluid tight relationship with the hollow fiber membranes and to ensure that the tube sheet can withstand any pressure differentials to which it may be subjected during the intended separation operation.
The performance demanded of a tube sheet will depend upon the anticipated operating conditions of the separation module. Modules containing hollow fiber membranes have found acceptance for use in desalination, ultrafiltration, and hemodialysis. In general, these separation operations provide relatively mild environments, i.e., the process streams contain little, if any, concentrations of moieties which may adversely affect the material of the tube sheet, that is, by loss of physical strength, or integrity, or by swelling. Moreover, in operations such as hemodialysis in which little of any pressure differential is exerted across the tube sheet, the strength of the tube sheet is not a prime consideration. Accordingly, a wide freedom of choice exists in selecting the resin for fabricating the tube sheet. For example, the aspects of strength and chemical resistance can be sacrificed to avoid high polymerization temperatures (e.g., exotherms), in order to provide a desirable fluid liquid resin for penetration into the previously assembled bundle of hollow fiber membranes, and to ensure good adherence of the tube sheet to the hollow fiber membranes. Even with such modules which operate under relatively mild conditions, considerable difficulty may still be experienced in obtaining a suitable resin for forming a tube sheet. These difficulties clearly become more severe when the tube sheet must exhibit high strength and chemical resistance.
For instance, in view of the benefits which can be provided by fluid separations affected by membranes, it is desired to provide tube sheets which will enable separation module technology to be utilized in harsher environments such as gaseous purge streams and liquid waste streams from, e.g., chemical plants or refineries, which can contain moieties which are often deleterious to resinous materials. Such tube sheets should exhibit high strengths in order to withstand the high pressure differentials (often exceeding 30 or 40 or even 60 atmospheres) which may be required to obtain advantageous permeate flux through the walls of the hollow fiber membranes. Moreover, the tube sheet should retain its strength and dimensional stability over the long periods of operation which are desired for separation modules.
A wide variety of resins have been proposed for preparing tube sheets for hollow fiber membranes. For instance, Geary, et al, in U.S. Pat. No. 3,499,062, issued Mar. 3, 1970, suggest the use of solders, cements, waxes, adhesives, natural and synthetic resins. McLain in U.S. Pat. No. 3,422,008, issued Jan. 14, 1969, disclose the use of epoxy resins for forming tube sheets and suggest that phenol-aldehyde resins, melamine-aldehyde resins, thermosetting artificial rubbers, and acrylic resins may also be suitable. Other materials which have been disclosed for use as materials for forming tube sheets include urethane resins, silicone resins, polysulfides, acetals, cellulosics, fluorocarbons, vinyls, styrenes, polyethylene and polypropylene. More recently, U.S. Pat. No. 4,323,453, issued Apr. 6, 1982, discloses tube sheets having a plurality of hollow fiber membranes suitable for fluid separations which are adapted to provide a fluid tight relationship within a separation module comprised of a cured epoxy resin, for example, polyglycidyl resin and an imidazole curing agent.
Generally, tube sheets are fabricated using a resin, which may be a natural or synthetic resin. The resin can be applied to the hollow fiber membranes which are then assembled into a bundle or can be cast around a previously assembled bundle of hollow fiber membranes as a liquid and then solidified, e.g., by curing. Curing of the tube sheets which include a substantial volume of material generally results in shrinkage during-solidifying from the liquid state. These curing and shrinking tube sheets enhance the possibility of future fluid leakage around the peripheral of the tube sheet, i.e., between the tube sheet and the cylindrical shell in which the tube sheet is formed.